The production of air products in a liquid or gaseous state by the cryogenic fractionation of air in air fractionation plants is known and described, for example, in H.-W. Häring (ed.), Industrial Gases Processing, Wiley-VCH, 2006, in particular section 2.2.5, “Cryogenic Rectification”.
Air fractionation plants have distillation column systems which may for example take the form of two-column systems, in particular of conventional Linde double-column systems, but also of three- or multi-column systems. In addition to the distillation columns for obtaining nitrogen and/or oxygen in a liquid and/or gaseous state (for example liquid oxygen (LOX), gaseous oxygen (GOX), liquid nitrogen (LIN) and/or gaseous nitrogen (GAN)), i.e. the distillation columns for nitrogen-oxygen separation, distillation columns may be provided for obtaining further air components, in particular the noble gases krypton, xenon and/or argon.
The present invention is in particular intended for use in air fractionation plants, in which oxygen-rich streams are withdrawn from the distillation column system for nitrogen-oxygen separation predominantly or exclusively in a gaseous state. The invention may. however, also be used in air fractionation plants in which liquid streams are withdrawn from the distillation column system to provide oxygen-rich products, for example in air fractionation plants with internal compression, providing that enrichment of components which are higher-boiling than oxygen as explained below is possible. Air fractionation plants with internal compression are explained for example in loc. cit., section 2.2.5.2, “Internal Compression”.
The distillation column systems of typical air fractionation plants are operated at various operating pressures in their distillation columns. Known double-column systems have, for example, a “high-pressure” column (also simply designated pressure column) and a “low-pressure” column. The operating pressure of the high-pressure column amounts for example to 4.3 to 6.9 bar, preferably approx. 5.0 bar. The low-pressure column is operated at an operating pressure of for example 1.3 to 1.7 bar, preferably approx, 1.5 bar, The purpose of having an operating pressure of the low-pressure column which is slightly above atmospheric is essentially to be able to withdraw products from a corresponding air fractionation plant without using additional pumps. The pressures stated here and hereinafter are absolute pressures.
As is known, the air fed into the high-pressure column of a double-column system is used to obtain an oxygen-enriched bottom product (also designated enriched liquid) which is transferred into the low-pressure column. In the low-pressure column, a bottom product predominantly containing oxygen is separated from the oxygen-enriched bottom product from the high-pressure column and optionally further streams fed into the low-pressure column. In order to provide an ascending gas stream in the low-pressure column and so maintain rectification, the bottom product of the low-pressure column is continuously heated, such that a proportion of the bottom product is continuously passing into the gas phase. Heating may proceed in an internal or external condenser-vaporizer, also designated main condenser, which is heated with a gaseous, nitrogen-rich top product from the high-pressure column.
It may here be problematic that, during the described operation of the low-pressure column, less readily volatile components originating from the oxygen-enriched bottom product from the high-pressure column and thus ultimately from the fed air and any further streams fed into the low-pressure column may overtime be enriched in the bottom thereof or in a vaporization chamber of a corresponding external condenser-vaporizer. Components which may be considered critical in this connection are in particular hydrocarbons with up to four carbon atoms, as well as compounds such as nitrous oxide and carbon dioxide, which cannot be completely separated from the feed air using ordinary effort.
Maximum admissible concentrations of corresponding compounds are stated, for Example, in document 65/13/E from the Industrial Gas Council (IGC) of the European Industrial Gases Association (EIGA), Appendices E and F, “Maximum contaminant levels in liquid oxygen thermosyphon reboiler operation at 1.2 bar abs” and “Maximum contaminant levels in liquid oxygen downflow reboiler operation at 1.2 bar abs”. As is further explained therein in section 7.3.2, “Purging”, the problem of enrichment is less pronounced in plants in which a significant proportion of liquid or gaseous, internally compressed oxygen-rich products is obtained, because in this ease a proportion of the bottom product is continuously drawn off in liquid form from the bottom of the low-pressure column or a vaporization chamber of a corresponding external condenser-vaporizer. However, in air fractionation plants in which only already gaseous, oxygen-rich streams are withdrawn from the low-pressure column, it is in contrast necessary, preferably continuously, to draw off a small proportion of the bottom product as a “scavenging volume”. The cited EIGA publication here proposes 0.1 to 0.2% of the introduced air volume. If continuous withdrawal off is not possible, a suitable volume should be withdrawn intermittently, i.e. at least every 12 hours.
Corresponding enrichment of unwanted components may also occur in the top condensers of known air fractionation plants with distillation columns for obtaining argon, the vaporization chamber of which is charged with an oxygen-enriched bottom product from the high-pressure column. The same also applies to the bottoms of krypton/xenon enrichment columns, as explained below. In general corresponding problems occur whenever a liquid volume is fed in a vaporization chamber by means of a cryogenic oxygen-rich liquid and a proportion of the liquid volume is continuously transferred into the gas phase by vaporization, in particular when no appreciable proportions are withdrawn from the liquid volume in liquid form.
In practice, difficulties may arise in adjusting the volume, i.e. the scavenging volume, which is drawn off in liquid form from corresponding vaporization chambers, it is accordingly desirable for economic reasons to keep this volume as small as possible, since it can then typically be put to no further use and is therefore lost from the process. Furthermore, refrigeration losses inevitably occur when cryogenic liquids are discharged without passing through heat exchangers. On the other hand, if the volume is too small the mentioned components are excessively enriched in the vaporization chambers.
There is therefore a requirement tor a simple and reliable way of ascertaining the enrichment of components which are higher-boiling than oxygen in an oxygen-rich, cryogenic liquid, such that a simple and inexpensive way is available which permits verification of compliance with specifications for and/or adjustment of volumes to be withdrawn in this manner.